The present invention relates to thermal transfer of organic materials from a donor element to a receiving device such as an OLED device.
In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media are required to produce the RGB pixels. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
A suitable method for patterning high-resolution OLED displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. this method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable color-forming organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. Another problem is that the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily. Littman and Tang (commonly-assigned U.S. Pat. No. 5,688,551) teach the patternwise transfer of organic EL material from an unpatterned donor sheet to an EL substrate. A series of patents by Wolk et al (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teaches a method that can transfer the luminescent layer of an EL device from a donor element to a substrate by heating selected portions of the donor with a laser beam. Each layer is an operational or non-operational layer that is utilized in the function of the device.
In these processes a donor containing the electroluminescent materials is heated by radiation and transferred to a receiver which may already contain a portion of the active device. The device may then be finished by the application of further layers. This process allows the patterning of colors by the use of a suitable donor which contains an electron or hole conductors host and a dopant. The final light emitting device must have the dopant mixed together to give a good emission. It is difficult to co-evaporate two or more materials simultaneously and maintain a constant controlled ratio. The resulting emission from these radiation-transferred devices also have need for improved efficiency.
It is well known to those skilled in the field of OLED device fabrication, that control of moisture is critical in order to obtain devices of high quality, efficiency, and stability. Most materials which are thermally suitable for use as supports for donor elements will come to equilibrium with 0.1% to 1.0% of water by weight at room temperature in the presence of 100% relative humidity, or when immersed in water. The presence of this moisture in the support material can lead to inferior OLED devices if it is not meticulously controlled. The presence of this water in the donor element can compromise the coating of metallic absorbing layers onto the support, the coating of organic transfer layers onto the support, and the transfer of organic transfer layers onto the OLED device.
It is therefore an object of the present invention to reduce the adverse effects of moisture inherent to laser transfer donor elements which are moisture sensitive, such as those used in the fabrication of OLED devices. This object is achieved by providing a low moisture donor element for use in transferring moisture sensitive organic material to an OLED device, comprising:
(a) a low moisture transparent support;
(b) an optional absorber antireflecting layer which increases the absorption efficiency of the element;
(c) a metallic heat absorbing layer provided over the low moisture transparent support; and
(d) an organic transfer layer consisting of one or more heat transferable organic layers provided over the support.
Failure to remove moisture from the transparent support material results in a greater tendency for the laser exposure to ablate the absorber material, thereby contaminating the OLED device. Furthermore, moisture escaping from the transparent support during the organic coating step is very likely to result in degradation of the organic layers. The delay required to allow the moisture to be removed by exposure to vacuum in the actual organic coating station is unacceptable to a production environment, and there is no assurance that adequate moisture has been removed. It is also likely that even if ablation is avoided during the laser writing step, that the moisture released when the support is heated by the laser could adversely impact the OLED device.
Use of the transparent supports which can absorb moisture requires costly handling, and accidental exposure to moisture could go undetected until inferior OLED product is fabricated. Also, as a product, these handling restrictions could be a great inconvenience to the customer. The low moisture transparent supports do not suffer from complicated handling and storage requirements, but adhesion problems could limit the operating space for use product made from these materials.